1. Field of the Invention
This invention generally relates to methods and apparatus for driving passive, electro-optic displays with greater efficiency. The invention is particularly suitable for driving passive matrix organic light emitting diode displays.
2. Related Technology
Organic light emitting diodes (OLEDs) comprise a particularly advantageous form of electro-optic display. They are bright, colourful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic LEDs may be fabricated using either polymers or small molecules in a range of colours (or in multi-coloured displays), depending upon the materials used Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507.
A basic structure 100 of a typical organic LED is shown in FIG. 1a. A glass or plastic substrate 102 supports a transparent anode layer 104 comprising, for example, indium tin oxide (ITO) on which is deposited a hole transport layer 106, an electroluminescent layer 108, and a cathode 110. The electroluminescent layer 108 may comprise, for example, a PPV (poly(p-phenylenevinylene)) and the hole transport layer 106, which helps match the hole energy levels of the anode layer 104 and electroluminescent layer 108, may comprise, for example, PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene). Cathode layer 110 typically comprises a low work function metal such as calcium and may include an additional layer immediately adjacent electroluminescent layer 108, such as a layer of aluminium, for improved electron energy level matching. Contact wires 114 and 116 to the anode the cathode respectively provide a connection to a power source 118. The same basic structure may also be employed for small molecule devices.
In the example shown in FIG. 1a light 120 is emitted through transparent anode 104 and substrate 102 and such devices are referred to as “bottom emitters”. Devices which emit through the cathode may also be constructed, for example by keeping the thickness of cathode layer 110 less than around 50-100 nm so that the cathode is substantially transparent.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixelated display. A multicoloured display may be constructed using groups of red, green, and blue emitting pixels. In such displays the individual elements are generally addressed by activating row (or column) lines to select the pixels, and rows (or columns) of pixels are written to, to create a display. Either a passive matrix or an active matrix configuration may be employed. Broadly speaking in a passive matrix display a pixel driver such as a constant current driver is multiplexed onto a pixel whereas in an active matrix display a dedicated driver is provided for each pixel. Thus so-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned, somewhat similarly to a TV picture, to give the impression of a steady image.
FIG. 1b shows a cross section through a passive matrix OLED display 150 in which like elements to those of FIG. 1a are indicated by like reference numerals. In the passive matrix display 150 the electroluminescent layer 108 comprises a plurality of pixels 152 and the cathode layer 110 comprises a plurality of mutually electrically insulated conductive lines 154, running into the page in FIG. 1b, each with an associated contact 156. Likewise the ITO anode layer 104 also comprises a plurality of anode lines 158, of which only one is shown in FIG. 1b, running at right angles to the cathode lines. Contacts (not shown in FIG. 1b) are also provided for each anode line. An electroluminescent pixel 152 at the intersection of a cathode line and anode line may be addressed by applying a voltage between the relevant anode and cathode lines.
Referring now to FIG. 2a, this shows, conceptually, a driving arrangement for a passive matrix OLED display 150 of the type shown in FIG. 1b. A plurality of constant current generators 200 are provided, each connected to a supply line 202 and to one of a plurality of column lines 204, of which for clarity only one is shown. A plurality of row lines 206 (of which only one is shown) is also provided and each of these may be selectively connected to a ground line 208 by a switched connection 210. As shown, with a positive supply voltage on line 202, column lines 204 comprise anode connections 158 and row lines 206 comprise cathode connections 154, although the connections would be reversed if the power supply line 202 was negative and with respect to ground line 208.
As illustrated pixel 212 of the display has power applied to it and is therefore illuminated. To create an image connection 210 for a row is maintained as each of the column lines is activated in turn until the complete row has been addressed, and then the next row is selected and the process repeated. Alternatively a row may be selected and all the columns written in parallel, that is a row selected and a current driven onto each of the column lines simultaneously, to simultaneously illuminate each pixel in a row at its desired brightness. Although this latter arrangement requires more column drive circuitry it is preferred because it allows a more rapid refresh of each pixel. In a further alternative arrangement each pixel in a column may be addressed in turn before the next column is addressed, although this is not preferred because of the effect, inter alia, of row resistance. It will be appreciated that in the arrangement of FIG. 2a the functions of the column driver circuitry and row driver circuitry may be exchanged.
The skilled person will understand that where the term “brightness” is employed, when it is applied to an OLED it should generally be taken to mean luminance.
It is usual to provide a current-controlled rather than a voltage-controlled drive to an OLED because the brightness or more precisely, luminance, of an OLED is determined by the current flowing through it, this determining the number of photons it outputs. Thus the brightness-current curve of an OLED is broadly linear whereas the brightness-voltage curve is strongly non-linear. For this reason, in a voltage-controlled configuration the brightness can vary across the area of a display and with time, temperature, and age, making it difficult to predict how bright a pixel will appear when driven by a given voltage. In a colour display the accuracy of colour representations may also be affected.
FIGS. 2b to 2d illustrate, respectively, the current drive 220 applied to a pixel, the voltage 222 across the pixel, and the light output 224 from the pixel over time 226 as the pixel is addressed. The row containing the pixel is addressed and at the time indicated by dashed line 228 the current is driven onto the column line for the pixel. The column line (and pixel) has an associated capacitance and thus the voltage gradually rises to a maximum 230. The pixel does not begin to emit light until a point 232 is reached where the voltage across the pixel is greater than the OLED diode voltage drop. Similarly when the drive current is turned off at time 234 the voltage and light output gradually decay as the column capacitance discharges. Where the pixels in a row are all written simultaneously, that is where the columns are driven in parallel, the time interval between times 228 and 234 corresponds to a line scan period.
It is desirable for many applications to be able to provide a greyscale-type display, that is one in which the apparent brightness of individual pixels may be varied rather than simply set either on or off. Here “greyscale” refers to such a variable brightness display, whether a pixel is white or coloured.
The conventional method of varying pixel brightness is to vary pixel on-time using Pulse Width Modulation (PWM). In the context of FIG. 2b above the apparent pixel brightness may be varied by varying the percentage of the interval between times 228 and 234 for which drive current is applied. Normally in a PWM scheme a pixel is either full on or completely off but the apparent brightness of a pixel varies because of time integration within the observer's eye.
Pulse Width Modulation schemes provide a good linear brightness response but to overcome effects related to the delayed pixel turn-on they generally employ a pre-charge current pulse (not shown in FIG. 2b) on the leading edge 236 of the driving current waveform, and sometimes a discharge pulse on the trailing edge 238 of the waveform. This can improve the greyscale resolution but at the expense of increased power consumption. As a result, charging (and discharging) the column capacitance can account for roughly half the total power consumption in displays incorporating this type of brightness control. Other significant factors which the applicant has identified as contributing to the power consumption of a display plus driver combination include dissipation within the OLED itself (a function of OLED efficiency), resistive losses in the row and column lines and the effects of limited current driver compliance, as explained in more detail later.
FIG. 3 shows a schematic diagram 300 of a generic driver circuit for a passive matrix OLED display. The OLED display is indicated by dashed line 302 and comprises a plurality n of row lines 304 each with a corresponding row electrode contact 306 and a plurality m of column lines 308 with a corresponding plurality of column electrode contacts 310. An OLED is connected between each pair of row and column lines with, in the illustrated arrangement, its anode connected to the column line. A y-driver 314 drives the column lines 308 with a constant current and an x-driver 316 drives the row lines 304, selectively connecting the row lines to ground. The y-driver 314 and x-diver 316 are typically both under the control of a processor 318. A power supply 320 provides power to the circuitry and, in particular, to y-driver 314. It will be appreciated that which electrodes are labelled as “row” electrodes and which are labelled as “column” electrodes is arbitrary.
FIG. 4 shows, schematically, a current driver 402 for one column line of a passive matrix OLED display, such as the display 302 of FIG. 3. Typically a plurality of such current drivers are provided in a column driver integrated circuit, such as Y-driver 314 of FIG. 3, for driving a plurality of passive matrix display column electrodes.
A particularly advantageous form of current driver 402 is described in the applicant's co-pending British patent application no. 0126120.5 entitled “Display Driver Circuits”. The current driver 402 of FIG. 4 outlines the main features of this circuit and comprises a current driver block 406 incorporating a bipolar transistor 416 which has an emitter terminal substantially directly connected to a power supply line 404 at supply voltage Vs. (This does not necessarily require that the emitter terminal should be connected to a power supply line or terminal for the driver by the most direct route but rather that there should preferably be no intervening components, apart from the intrinsic resistance of tracks or connections within the driver circuitry between the emitter and a power supply rail). A column drive output 408 provides a current drive to OLED 412, which also has a ground connection 414, normally via a row driver MOS switch (not shown in FIG. 4). A current control input 410 is provided to current driver block 406 and, for the purposes of illustration, this is shown connected to the base of transistor 416 although in practice a current mirror arrangement is preferred. The signal on current control line 410 may comprise either a voltage or a current signal.
The arrangement of FIG. 4 is useful because the (optionally variable) current generator has a high compliance, that is a low value of Vs-Vo, where Vs is the supply voltage and Vo is substantially the maximum output voltage of the current source. The lower the current driver compliance (i.e. the greater Vs-Vo), the greater the power losses due to limited driver compliance. Further compliance-related techniques for reducing power consumption are described in the Applicant's UK Patent Application number 0213989.7 filed on 18 Jun. 2002.
Specific examples of OLED display drivers are described in U.S. Pat. No. 6,014,119, U.S. Pat. No. 6,201,520, U.S. Pat. No. 6,332,661, EP 1,079,361A and EP 1,091,339A; OLED display driver integrated circuits are also sold by Clare Micronix of Clare, Inc., Beverly, Mass., USA. The Clare Micronix drivers provide a current controlled drive and achieve greyscaling using a conventional PWM approach; U.S. Pat. No. 6,014,119 describes a driver circuit in which pulse width modulation is used to control brightness; U.S. Pat. No. 6,201,520 describes driver circuitry in which each column driver has a constant current generator to provide digital (on/off) pixel control; U.S. Pat. No. 6,332,661 describes pixel driver circuitry in which a reference current generator sets the current output of a constant current driver for a plurality of columns; and EP 1,079,361A and EP 1,091,339A both describe similar drivers for organic electroluminescent display elements in which a voltage drive rather than a current drive is employed.
Prior art techniques for reducing the power consumption of liquid crystal displays (LCDs) are described in U.S. Pat. No. 6,323,849 and EP 0 811 866A. U.S. Pat. No. 6,323,849 describes an LCD display with a partial display mode in which a control circuit controls display drivers to turn off a portion of the display which does not show useful information. When the LCD module is in a partial display mode the line frequency may also be reduced whilst maintaining the same frame refresh rate, allowing a lower voltage to be used to produce the same amount of charge. However, a user must predetermine which portion of the display is to be used, which will typically require additional control functions and software in the device for which the display is provided. EP 0 811 866A describes a similar technique, albeit with a more flexible driving arrangement. Another technique is described in the Applicant's UK patent application number 0209502.4.
U.S. Pat. No. 4,823,121 describes an electroluminescent (EL) panel driving system which detects the absence of a HIGH level signal representing a spot illumination of the EL panel in the image data of a line and, in response to this, prevents four circuits (a pre-charge circuit, a pullup circuit, a write-in circuit and a source circuit) from being activated. However the power savings provided by this technique are specific to the drive arrangement for the type of electroluminescent panel described and are not readily generalisable. Furthermore the savings are relatively modest.
It is generally desirable to reduce the power consumption of the display plus driver combination, particularly whilst retaining the ability to provide a variable brightness or “greyscale” display.